In thermal dye transfer printing, an image is formed on a receptor sheet by selectively transferring a dye to a receptor sheet from a dye donor sheet placed in momentary contact with the receptor sheet. Material to be transferred from the dye donor sheet is directed by a thermal printhead, which consists of small electrically heated elements (print heads). These elements transfer image-forming material from the dye donor sheet to areas of the dye receptor sheet in an image-wise manner. Thermal dye transfer systems have advantages over other thermal transfer systems, such as chemical reaction systems, thermal mass transfer systems, and sublimation dye transfer systems. In general thermal dye transfer systems offer greater control of gray scale than these other systems, but they have problems as well. One problem is release of the dye donor and receptor sheets during printing. This has been addressed often by the addition of dye-permeable release coatings applied to the surface of the dye receptor layer. Additionally, materials are required for use in the receptor layer having suitable dye permeability, mordanting properties, adhesion to the substrate, and long term light and thermal stability.
Polyvinyl chloride derivatives and copolymers have been heavily used in thermal dye transfer receptor sheets, because of their properties in these areas. For example, U.S. Pat. No. 4,853,365 discloses that chlorinated polyvinyl chloride, used as a dye receptor, has good dye solubility and high dye receptivity. Similarly, vinyl chloride/vinyl acetate copolymers have also been used in thermal dye transfer receptor sheets as described in Japanese published application nos. 29,391 (1990) and 39,995 (1990). Japanese published application no. 160,681 (1989) discloses dye acceptance layers comprising polyvinyl chloride-polyvinyl alcohol copolymers, and Japanese published application nos. 43,092 (1990), 95,891 (1990) and 108,591 (1990) discloses dye image receiving layers comprising a hydroxy modified polyvinyl chloride resin and an isocyanate compound. U.S. Pat. No. 4,897,377 discloses a thermal transfer printing receiver sheet comprising a supporting substrate coated on at least one surface with an amorphous polyester resin. Published European patent application 133,012 (1985) discloses a heat transferable sheet having a substrate and an image-receiving layer thereon comprising a resin having an ester, urethane, amide, urea, or highly polar linkage, and a dye-releasing agent, such as a silicone oil, being present either in the image-receiving layer or as a release layer on at least part of the image receiving layer. Published European patent application 133,011 (1985) discloses a heat transferable sheet based on imaging layer materials comprising first and second regions respectively comprising (a) a synthetic resin having a glass transition temperature of from -100.degree. to 20.degree. C., and having a polar group, and (b) a synthetic resin having a glass transition temperature of 40.degree. C. or above.
Generally, polyvinyl chloride based polymers are photolytically unstable, decomposing to form hydrogen chloride, which in turn degrades the image-forming dyes. This has made necessary the extensive use of UV stabilizers and compounds that neutralize hydrogen chloride. The dye transfer receptor sheets of this invention employ a modified polyvinyl chloride resin that has much higher light stability than materials previously used, while retaining the desirable properties associated with polyvinyl chloride based resins.
What the background art does not disclose but this invention teaches is that epoxy/hydroxy/sulfonate functionalized polyvinyl chloride resins are particularly useful components in the construction of thermal dye transfer receptor sheets having improved dye image stability.